1. Field of the Invention
The present invention relates to a toner appropriate for an electrophotographic process of a copier, a printer or a facsimile and an image-forming method using the same. More specifically, it relates to an electrophotographic toner which is preferably used in a color copier.
2. Description of the Related Art
As a photographic method, many methods are known as described in Japanese Patent Publication No. 23910/1967. Generally, a fixed image is formed through plural steps of electrically forming a latent image on a photoreceptor using a photoconductive material by various methods, developing this latent image with a toner, transferring the toner latent image of the photoreceptor on a material for transfer such as paper through or not through a middle transfer body and then fixing this transferred image by heating, pressing or heat-pressing or with a solvent steam. The toner remaining on the photoreceptor is cleaned by various methods as required, and the plural steps are repeated.
In this fixing technique, a heating roller fixing method in which a transfer body having a toner image is inserted between a pair of rollers, a heating roller and a pressure roller for fixing is a general method. Further, as the same type of the technique, changing one or both of the rollers to belt(s) is also known.
In these methods, in comparison with other fixing methods, a fast fixed image is obtained at high speed, an energy efficiency is high, and the environmental pollution owing to evaporation of a solvent is reduced. However, since the toner image is brought into direct contact with rollers or belts, so-called offset in which a part of the toner is adhered to rollers or belts tends to occur in the fixing. Especially when a temperature of a fixing device is high, a cohesive force of a molten toner is decreased, and the offset is liable to occur.
Further, in order to decrease an amount of energy used, it is required that the fixing can be conducted at a lower temperature. Especially, in recent years, it is desired that the current passage through the fixing device is stopped except when it is used for complete energy saving. Thus, the temperature of the fixing device is required to be elevated to a use temperature immediately after the current passage. To this end, it is preferable to minimize a heat capacity of the fixing device. However, in this case, an amplitude of the temperature of the fixing device tends to be increased more than as usual. That is, the overshoot of the temperature after the start-up of the current passage is increased, while the temperature is decreased owing to passage of paper. Further, when paper having a smaller size than the fixing device is continuously passed, a difference in temperature between a paper passage portion and a non-paper-passage portion is also increased. Especially, in a high-speed copier or printer, a power capacity is sometimes insufficient, and this tendency is strong. Accordingly, a toner having a so-called wide fixing latitude in which the fixing is conducted at a low temperature and offset does not occur up to a high temperature region has been in demand.
In order to decrease the fixing temperature of the toner, the use of a crystalline resin as a binder resin is proposed in Japanese Patent Publication Nos. 24702/1992 and 24703/1992 and Japanese Patent Laid-Open No. 329917/1997. Although this method can decrease the fixing temperature, an offset resistance is not necessarily satisfactory. That is, since the molten toner permeates paper, occurrence of offset can be prevented to some extent. However, there arises a problem that since the molten toner permeates paper too much, a uniform high-density image is not obtained.
Meanwhile, for preventing occurrence of offset, it is known that a resin having an appropriate molecular weight distribution which is obtained by blending a low polymer with a high polymer is used as a binder resin of a toner (Japanese Patent Laid-Open No. 134652/1975). Also known is the use of a crosslinked resin (Japanese Patent Publication No. 23354/1976). Nevertheless, these methods cannot provide a fixing latitude which is wide enough to meet the needs in recent years. Offset hardly occurs by using a large amount of a high polymer or a crosslinked polymer, but the fixing temperature is increased. Meanwhile, when a molecular weight of a low polymer is decreased or an amount thereof is increased for decreasing the fixing temperature, a temperature at which offset occurs is decreased. Further, the fixing temperature can also be decreased by decreasing a glass transition temperature of a binder resin or using a plasticizer. However, a so-called blocking phenomenon of cohering and solidifying a toner during storage or in a developing device occurs.
In order to solve these problems, there are a large number of proposals on a technique in which a crystalline polymer is used as a binder resin not singly but in combination with an amorphous polymer as described below. For example, the combined use of a crystalline polymer and an amorphous polymer is described in Japanese Patent Laid-Open No. 79860/1990. A polymer obtained by chemically combining a crystalline polymer with an amorphous polymer is described in Japanese Patent Laid-Open Nos. 163756/1989, 163757/1989, 81770/1992, 155351/1992 and 44032/1993.
However, when the amount of the amorphous polymer is larger than that of the crystalline polymer, the amorphous polymer is a continuous phase, and the crystalline polymer is a disperse phase. In this case, since the crystalline polymer is covered with the amorphous polymer, the crystalline polymer is not problematic. Meanwhile, since the melting of the overall toner is controlled by the softening point of the amorphous polymer, a low-temperature fixing property is not provided.
As the contradictory characteristics, namely, the decrease in the fixing temperature and the prevention of offset are thus required, a toner that actually satisfies well the characteristics has not yet been obtained.